1. Field of the Invention
The present invention relates to a color imaging element, and particularly, to a color imaging element that can decrease generation of color moire and increase resolution.
2. Description of the Related Art
An output image of a single-plate color imaging element is a RAW image (mosaic image). Therefore, a multi-channel image is obtained by a process of interpolating (demosaicing processing) a pixel of a missing color from a surrounding pixel. In this case, there is a problem in reproduction characteristics of a high-frequency image signal. Compared to a black and white imaging element, aliasing easily occurs in an image taken by a color imaging element, and it is important to expand a reproduction band to increase resolution while decreasing generation of color moire (false color).
The demosaicing process is a process of calculating all color information of each pixel from a mosaic image corresponding to a color filter array of the single-plate color imaging element and is also called synchronization processing. For example, in a case of an imaging element including color filters of three RGB colors, the color information of all of RGB is calculated in the process for each pixel from the mosaic image made of RGB.
A primary-color Bayer array as a color array most widely used in the single-plate color imaging element includes green (G) pixels arranged in a check pattern and red (R) and blue (B) arranged line-sequentially. Therefore, G signals have a problem of reproduction accuracy in generation of high frequency signals in oblique directions, and R and B signals have a problem of reproduction accuracy in generation of high frequency signals in horizontal and vertical directions.
A black and white vertical-striped pattern (high frequency image) as shown in (A) portion of FIG. 19 enters an imaging element in a Bayer array shown in (B) portion of FIG. 19, and the pattern is sorted into Bayer color arrays to compare the colors. As shown in (C) to (E) portions of FIG. 19, R forms a light and flat color image, B forms a dark and flat color image, and G forms a light and dark mosaic color image. Although there is no density difference (level difference) between RGB with respect to the original black and white image, the image is colored depending on the color array and the input frequency.
Similarly, a black and white oblique high frequency image as shown in (A) portion of FIG. 20 enters an imaging element in a Bayer array shown in (B) portion of FIG. 20, and the image is sorted into Bayer color arrays to compare the colors. As shown in (C) to (E) portions of FIG. 20, R and B form light and flat color images, while G forms a dark and flat color image. Assuming that the value of black is 0 and the value of white is 255, the black and white oblique high frequency image turns green, because only G is 255. In this way, an oblique high frequency image cannot be correctly reproduced in the Bayer array.
In the imaging apparatus using the single-plate color imaging element, an optical low-pass filter formed by an anisotropic substance such as crystal is generally arranged on the front side of the color imaging element to prevent optically reducing the high frequency wave. However, although the coloring caused by folding of the high frequency signal can be reduced in the method, there is a problem that the resolution is reduced accordingly.
To solve the problem, a color imaging element is proposed, wherein a color filter array of the color imaging element is a three-color random array satisfying array restrictions in which an arbitrary target pixel is adjacent to three colors including the color of the target pixel on four sides of the target pixel (Japanese Patent Application Laid-Open No. 2000-308080; PTL 1).
An image sensor of a color filter array is also proposed, wherein the image sensor includes a plurality of filters with different spectral sensitivity, and first and second filters among the plurality of filters are alternately arranged in a first predetermined period in one of the diagonal directions of a pixel grid of the image sensor and are alternately arranged in a second predetermined period in the other diagonal direction (Japanese Patent Application Laid-Open No. 2005-136766; PTL 2).
A color array is further proposed, wherein in a color solid-state imaging element of three primary colors of RGB, sets of three pixels including horizontally arranged R, G, and B are arranged in a zigzag manner in the vertical direction to make appearance frequencies of RGB equal and to cause arbitrary lines (horizontal, vertical, and oblique lines) on an imaging plane to pass through all colors (Japanese Patent Application Laid-Open No. 11-285012; PTL 3).
Furthermore, a color imaging element is also proposed, wherein R and B among the three primary colors of RGB are arranged every three pixels in horizontal and vertical directions, and G is arranged between R and B (Japanese Patent Application Laid-Open No. 8-23543; PTL 4).
A color imaging element is also proposed, wherein pixels are arranged in a diagonal grid shape (adjacent pixels on horizontal lines are arranged at a ½ pixel pitch), and lines with only G pixels and lines with repeated R, B pixels or B, R pixels are alternately arranged (Japanese Patent Application Laid-Open No. 10-136391; PTL 5).
A color imaging element is further proposed, the color imaging element including a plurality of pixels formed by photoelectric conversion elements arranged in a square grid shape and including a first pixel group and a second pixel group arranged in a checked grid shape, wherein color filters in the same color filter array as in PTL 5 are arranged for the first and second pixel groups (Japanese Patent Application Laid-Open No. 2009-60342; PTL 6). According to the imaging element, an image corresponding to the first pixel group and an image corresponding to the second group can be acquired at the same time by changing the exposure conditions, and the images can be combined to generate a wide dynamic image.